Correcting typewriters have long been known which could remove incorrect characters and allow entry of correct ones by following an appropriate, but lengthy, sequence of keystrokes. With the advent of recording typewriters (paper tape, magnetic tape, magnetic card, etc.) it became possible to reduce the number of keystrokes required, but insertion of additional characters was still a cumbersome procedure, generally prohibitively expensive in terms of time (patching paper tapes, transferring the "good" portion of an error-containing magnetic record to a new record, then skipping defective material, inserting the correct material and resuming recording of "good" material from the original record, etc.).
"Dictionary" typewriters (e.g. machines incorporating the structures of References I) have recently appeared on the market which contain an electronic memory for one or more pages of entered text stored in a random access section of the memory, together with a listing of a large number of commonly-used words--35,000, say--plus about 300 special words chosen by the typist because of their frequent use, all of these words being stored in a read-only section of the memory; and a comparison unit monitoring successive text entries and emitting a visual or auditory signal when comparison of an entered word with the listing of words in memory (the "dictionary") indicates a disagreement in the spelling of the entered word. That signal indicates that the typist's entry is erroneous and the spelling of the last-entered word should be checked in a printed dictionary. In more powerful versions where the typewriter has an associated display unit, the signal may visually indicate the incorrect word and a marker may be located at the site of the error in the incorrect word.
Dictionary equipment of the foregoing kind is also known which provides the further ability to display one-by-one a succession of correctly spelled words based on a sequence of initial characters entered by the typist (one such corrective procedure is that incorporated in the Smith Corona XD 7000 and the Smith Corona PWP 12, a dictionary typewriter and a word processing system, respectively; both manufactured by the present assignee and the dictionary/listing structure essentially corresponding to that disclosed in the applications cited initially). The typist's acceptance of one of the displayed words then results in printing of that desired word with erasure of the incorrect word (and subsequent characters or words, if any) previously printed on the record sheet (accompanied by reprinting of any subsequent characters or words also erased).
While the foregoing improvements have done much to enhance the quality of communications prepared by poor spellers, the efficiency of typewriter use is still not optimum because time spent looking up the correct spelling of a word in a printed dictionary or scanning a list of valid words successively presented in alphabetic order on a display would be more productively spent as further typing. Thus, there is need for reducing correction of errors to a simple, fully automatic operation, preferably by depression of just one key and by selection of a first-applied correction information in accordance with the kinds of errors the typist is most likely to commit, not only initially but also as experience is gained.